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Introduction
Use of animal models for pre-clinical evaluation of vaccines against respiratory syncytial virus (RSV) and post-exposure therapies is of prime importance. The enhanced disease observed in babies (3-9months) vaccinated with formalin-inactivated RSV vaccine in the 1960s (FI-RSV) [1] provided a strong impetus to find a pre-clinical animal model that can recapitulate the vaccine-associated enhanced respiratory disease (VAERD) and be used for exploring the mechanism of VAERD and for evaluating novel RSV vaccines and novel therapeutics. The main animal models currently used include primates, bovine calves, mice, and cotton rats [2] . Since mice can develop FI-RSV mediated VAERD they were found to be a useful pre-clinical animal model for vaccine evaluation, in spite of the fact that they are not a natural host of RSV. In mice, a high intranasal inoculum is often required for intranasal infection and spread to the lungs [2] . A chimeric A2 strain of RSV with the fusion protein of RSV strain Line 19 was shown to exhibit enhanced viral loads, mucus, and airway dysfunction in BALB/c mice [3] .
The readout of RSV infection following viral challenge in animal model is very important as it could have a significant impact on the numbers of animals per group that are required to achieve statistical power for evaluation of efficacy of vaccines and antivirals. Viral loads in RSV-infected mice are usually measured in the lungs collected from sacrificed mice by plaque assay using lung extracts in susceptible target cells or by qRT-PCR using RNA extracted from homogenized lung tissues. Both approaches are laborious and time consuming and may sometimes provide misleading results. In addition, these approaches do not allow monitoring dissemination of the virus within the host and require large numbers of animals that need to be sacrificed at multiple time points to follow kinetics of viral replication [4] .
Bioluminescence imaging of live RSV-infected mice can provide an alternative approach in which every animal is served as its own control and viral replication is followed over the length of the experiment. This approach reduces the number of animals required to reach statistical power, reduces animal suffering, and provides insight on the virus replication in difficult to access areas such as nasopharyngeal area. Previously, we have used the live imaging...